Both the proximal and distal radioulnar joints are synovial joints. The proximal joint lies between the head of the radius and the radial notch of the ulna. The distal radioulnar joint is separated from the wrist by an articular disc that extends from the base of the ulnar styloid process to the radius.
The distal radioulnar joint is a pivot-joint formed between the head of the ulna and the ulnar notch on the lower end of the radius. The articular surfaces are connected together by the volar radioulnar ligament, the dorsal radioulnar ligament, and the articular disk. The volar radioulnar ligament is a narrow band of fibers extending from the anterior margin of the ulnar notch of the radius to the front of the head of the ulna. The dorsal radioulnar ligament extends between corresponding surfaces on the dorsal aspect of the articulation. The articular disk is triangular in shape, and is placed transversely beneath the head of the ulna, binding the lower ends of the ulna and radius firmly together. Its periphery is thicker than its center, which is occasionally perforated. It is attached by its apex to a depression between the styloid process and the head of the ulna; and by its base, which is thin, to the prominent edge of the radius, which separates the ulnar notch from the carpal articular surface. Its margins are united to the ligaments of the wrist-joint. Its upper surface, smooth and concave, articulates with the head of the ulna, forming an arthrodial joint; its under surface, also concave and smooth, forms part of the wrist-joint and articulates with the triangular bone and medial part of the lunate. Both surfaces are clothed by synovial membrane; the upper, by that of the distal radioulnar articulation, the under, by that of the wrist.
The radius articulates in pronation and supination on the distal ulna. The ulna, a relatively straight forearm bone linked to the wrist, translates dorsal-palmarly to accept the modestly bowed radius. Since the sigmoid fossa socket in most wrists is relatively flat, ligaments are required to support the distal ulna. These ligaments include the triangular fibrocartilage (TFC), the extensor carpi ulnaris (ECU) subsheath, and the ulnar collateral ligament complex. The stabilizing elements of the triangular fibrocartilage (TFC), extensor carpi ulnaris (ECU) subsheath, and the ulnar collateral complex are well recognized along with the importance of a distal ulna component (ulnar head) for transfer of compressive loads between the ulnar carpus and the distal ulna across the distal radioulnar joint. The distal radioulnar joint shares loading forces that occur with forearm rotation and gripping. The arc of pronation and supination averages 150 to 160 degrees with the most useful portion being between 80 degrees pronation and 45 degrees supination.
One of the most common fractures in humans is fracture of the distal radius. Inherent bony instability, soft tissue damage, and frequent associated injuries make the distal radius fractures very difficult to treat. Distal radius fractures are usually caused by a fall on an outstretched hand. When a person falls on an outstretched hand, the hand suddenly becomes rigid, and the momentum from the fall will cause both a twisting force and a compressing force on the forearm. The kind of injury these forces are likely to cause depends on the age of the person who is injured. In children, and in older adults, such a fall is likely to result in a fracture of the radius. Distal radius fractures may also result from direct trauma such as might occur during an auto accident.
There are several types of fractures. A non-displaced fracture is one in which the bone cracks and the broken pieces stay in alignment. A torus or ripple fracture bends the back of the radius away from the growth plate. A displaced fracture is one in which the bone breaks in two or more pieces that move out of alignment. Such a break may be extremely painful and produces a deformity that is easily seen. An open or compound fracture is one in which the ends of the bone are displaced and pierce the skin. In these cases, there is a significant risk of infection.
For displaced broken bones to heal properly and without serious complications, they need to be set and held in place for the body to repair and replace the damaged bone. The process usually takes between 4 and 12 weeks. Some fractures may be set without surgery, the bones being held in place first with a splint and then, after healing has started, with a cast. If the bones are seriously displaced, however, or if there is damage that needs to be repaired, surgery may be needed and the bones may need to be held together with pins or wire.
Closed treatment methods including casting, pins and plaster, and external fixation have frequently yielded unsatisfactory results. Treatment using formal open reduction and internal fixation with the conventional plate system, when achieving anatomic reduction and early mobilization, has produced some promising results. The value of immediate mobilization of the injured joints is clear.
With distal radial fractures, muscles may gradually weaken from lack of use during bone healing. A patient may need physical therapy in order to regain proper use of the wrist.
Ligament disruption, ulnar styloid fractures, and fractures into the distal radioulnar joint are common occurrences following fractures of the distal radius and other rotational instability injuries of the forearm. Fracture or dislocation involving the distal radioulnar joint often results in a loss of forearm rotation related to either instability or incongruity between the sigmoid fossa of the distal radius and the ulnar head. A variety of different fractures involving the distal radius may cause this condition including the Colles' fracture and the Galeazzi fractures.
When there is loss of stability of the distal radioulnar joint, there is subsequent weakness in grip and pinch as well as potential loss of forearm rotation. Instability may also be associated with either an injury to the triangular fibrocartilage or to the ulnar styloid. When instability is present, a number of ligament reconstructive procedures have been devised to assist in treating the unstable distal ulna. Unfortunately, ligament reconstruction of the distal ulna is often incomplete in restoring stability, and joint replacement is often necessary.
Where there is an incongruity of the joint surface involving either the articulation of the ulnar head with the sigmoid fossa of the distal radius, or if there is a significant ulnar impaction syndrome between the distal articular surface of the head of the ulna and the ulna carpus, a joint replacement may be necessary. Specifically, this may include either joint replacement of the distal ulna or operative procedures designed to shorten the ulna or resect all or part of the distal ulna (i.e. Darrach, Bowers, or matched resection procedures).
Implants or prostheses are employed for restoring damaged upper and lower extremity bones such as fingers, wrists, elbows, knees and ankles of human patients. These prostheses are especially useful in the reconstruction of joints which, for example, have been damaged by pathological conditions such as rheumatoid arthritis, degenerative arthritis, aseptic necrosis, and for treating trauma which may have a debilitating effect on articular joints.
There are three types of arthroplasties: 1) unconstrained, 2) semi-constrained and 3) fully constrained. A common flaw with all of these current joint replacement designs is the inability to reconstruct and re-attach the replaced joint's vital native capsular and ligamentous restraints, which dictate, in large measure, the behavior and stability of the joint (i.e., its kinematics).
The primary reasons for wrist replacement surgery are to relieve pain and to maintain function in the wrist and hand. The primary indications, therefore, for reconstruction of the distal radioulnar joint by prosthetic replacement (ulnar head replacement only) are generally related to a fracture of the distal ulna or a fracture extending into the distal radioulnar joint producing post-traumatic arthritis. Degenerative arthritis from other causes is also a primary indication. This is considered if there is associated arthritis and an ulnar shortening procedure is contraindicated. Osteoarthritis, the most common form of arthritis, results from a gradual wearing away of the cartilage covering on bones. A third condition for primary ulna replacement is rheumatoid arthritis with a painful and unstable distal radioulnar joint. Rheumatoid arthritis is a chronic inflammatory disease of the joints that results in pain, stiffness and swelling. Rheumatoid arthritis usually affects several joints on both the right and left sides of the body. Both forms of arthritis may affect the strength of the fingers and hand, making it difficult to grip or pinch. In some cases, fusing the wrist bones together will reduce or eliminate pain and improve grip strength. However, if the bones are fused together, the ability of the wrist to move and bend is lost. Wrist replacement surgery may enable retention or recovery of wrist movements. In these situations, prosthetic replacement of the distal ulna with soft tissue advancement may be beneficial.
A distal ulnar prosthesis is also suitable to correct a previous resection of the distal ulna that has failed. Such will be the case for both partial resection of the joint articular surface and complete resection of the distal ulna. When faced with failed distal ulna resection, one has options towards reconstruction without restoring the distal radioulnar joint (DRUJ). For example, a failed distal ulna may be corrected by a pronator quadratus interposition, or, if there has been only a partial resection, a fusion of the distal radioulnar joint combined with a proximal pseudarthrosis (Suave-Kapandji procedure). These procedures, however, do not restore the normal DRUJ function of motion or load transfer and may be associated with instability of the distal ulna and proximal impingement of the ulna on the distal radius. In these cases, a distal ulna prosthesis is generally preferable. A distal ulnar prosthesis is also suitable to correct a previous prosthetic replacement such as a silicone ulnar head replacement which has failed.
A distal ulnar prosthesis attachable to a soft tissue pocket including the triangular fibrocartilage, ECU subsheath, and ulnar collateral ligament complex to thereby maintain distal radioulnar joint stability, which aligns anatomically with the sigmoid fossa of the distal radius and is isosymmetric with the anatomic center of rotation of the forearm, and that allows for a normal forearm rotation of approximately 150-170 degrees would be desirable. More specifically, it would be desirable to have such a modular distal ulnar prosthesis wherein there is no risk of separation of the two components (the stem and the head) due to biomechanical forces from the tissues attached by suture to the implant.